1. Field of the Invention
The invention relates to a method for increasing melt flow of bimodal ionomers. This invention also relates to trimodal ionomer compositions.
2. Description of Related Art
Thermoplastic polymers are commonly used to manufacture various shaped articles which may be utilized in applications such as automotive parts, food containers, signs, and packaging materials. Shaped articles comprising ionomers may be prepared from the molten polymer by a number of melt extrusion processes known in the art, such as injection molding, compression molding, blow molding, and profile extrusion.
Increasing melt flow, for example as measured by melt index (MI), may be important to melt processing of the ionomers for injection molding and thin-gauge coating applications. Lowering the neutralization or raising the base acid copolymer MI may provide higher melt flow for the ionomer, but this may compromise the performance of the material after it is formed into solid articles. For example, stiffness, toughness, scratch and scuff resistance, and melt strength may be affected.
Some strategies for increasing the melt index of ionomers have been described. For example, U.S. Pat. No. 5,929,174 describes neutralized ethylene acrylic acid copolymers that comprise a combination of divalent and monovalent cations and that exhibit melt index uplift. It is reported that ethylene methacrylic acid copolymers do not exhibit melt index uplift.
Bimodal ionomer compositions and their use in golf balls are described in U.S. Pat. Nos. 6,562,906; 6,762,246; 7,037,967 7,273,903 and 7,488,778 and U.S. patent application Ser. No. 12/315,731. The bimodal ionomers may also be used as scratch and scuff-resistant surface layers of a variety of articles (US Patent Appln. Publn. No. 2009/0130355). The bimodal ionomer compositions comprise an ethylene α,β-ethylenically unsaturated C3-8 carboxylic acid copolymer having weight average molecular weight (Mw) of about 80,000 to about 500,000 Da (high molecular weight copolymer) and an ethylene α,β-ethylenically unsaturated C3-8 carboxylic acid copolymer having (Mw) of about 2,000 to about 30,000 Da (low molecular weight copolymer).
It is desirable to increase the melt flow of bimodal ionomers while retaining good physical and mechanical properties for the solid material.